The earthquake resistance of small buildings can be increased by taking certain precautions and measures in site selection, building planning and construction as mentioned below:
1. site selection
building construction should be avoided
,a) near unstable embankments
,b) on sloping ground with pillars of different heights
,C) Flood Affected Areas
,D) with marked discontinuities like rock in some part and clay in part on sub-soil.
2. building plan
Symmetric schemes are more secure than asymmetric schemes. So go for square or rectangular plans instead of L, E, H, T shapes. The length of rectangular plans should not exceed twice the width.
The width of the foundation should not be less than 750 mm for one storey building and 900 mm for one storey building. The depth of foundation should not be less than 1.0 m for soft soil and 0.45 m for rocky ground. Remove all loose material including water from the trench and compact the bottom before pouring the foundation. After the foundation has been laid, back-fill the foundation properly and compactly.
In case of stone masonry
- Lay each stone flat on its wide face.
- Place the stones lengthwise across the thickness of the wall to ensure the inside and outside faces of the wall are interlocking.
- Fill in the gaps using small chips of stones with the minimum amount of mortar possible.
- Break the stone and make it angular so that it does not have a round face.
- Use through stones every 600 to 750 mm.
In case of brick masonry
- Use only properly burnt bricks.
- Lay the bricks along its groove marks to ensure a better bond with the next course.
In case of concrete blocks
- Keep the rough faces up and down to get a good bond.
- The block must be strong.
- Brush the top and bottom faces before laying.
In general, walls greater than 450 mm should be avoided. The length of the wall should be limited to 6 meters. Cross walls make the masonry stronger. It is better to build partition walls with main walls connecting the two.
5. door and window opening
- Walls with lots of doors and windows near each other quickly collapse.
- Windows should be placed on the same level.
- The total width of all openings in the wall should not exceed 1/3 of the length of the wall.
- Doors should not be placed at the end of the wall. They should be at least 500 mm away from the cross wall.
- The apparent width between the two openings should not be less than 600 mm.
- Use trusses instead of rafters in sloping roofs with spans more than 6 meters.
- A building with a 4-sided sloping roof is stronger than a double-sided sloping roof, as the gable walls tend to collapse quickly.
Limit visor or balcony projections to 0.9 m. Use beams and columns for larger projections.
Masonry parapet wall can easily collapse. It is better to make a parapet with bricks up to 300 mm after an iron railing.
9. concrete and mortar
Use river sand to make mortar and concrete. It should be filtered to remove the pebbles. The silt should be removed by holding it against the wind. Thick aggregates over 30 mm in size should not be used. Aggregates should be well graded and angular. Cement and aggregate should be dried thoroughly before watering.
The following RC bands should be provided
,a) plinth band
,b) lintel band
,C) roof band
,D) gable band.
The minimum thickness for making RC bands is 75 mm and at least twice the diameter of 8 mm are required. They should be fastened to steel limbs of 6 mm diameter at a distance of 150 mm.
If the wall size is large, diagonal and vertical bands can also be provided.
1 1 retrofitting
Retrofitting means designing a structure in a scientific manner so that all the elements of a building function as an integral unit.
This is generally the most economical and quickest way to achieve building security. Following are some of the methods of retrofitting:
- Anchor roof trusses for walls with brackets.
- Provide bracing at the level of the purlins and lower chord members of the truss.
- Reinforce the gable wall by attaching a sloping belt to the gable wall.
- Reinforce the corners with a seismic belt.
- The anchor attaches to the walls with floor brackets.
- Improve storey connections by providing vertical reinforcement.
- Induce tensile strength against vertical bending of walls by providing vertical reinforcement at all inner and outer corners.
- Enclose the wall opening with reinforcement.